I bet that Breitling did the same as Heuer. When the Cal 15 came out in 1971-72 they just used old stock cases. For example, a Heuer Autavia with cal. 12 is stamped correctly 1163 between the lugs. A Heuer Autavia caliber 15, ref 1563, is also stamped 1163 between the lugs. Why produce new cases if you had some stock ready? The cal. 15 was ment as a cheaper alternative to cal. 12. I don't think they used money to produce new cases.
I think the watch you are looking at should have the ref. 2111-15, the extra -15 indicating it's a cal.15 in a 2111 case.
See picture of another Breitling cal 15 case back. The -15 looks like it has been added later to a old stock case.
Just my 2cent
Cheers
Jarl
